The invention relates to a method for operating an internal combustion engine, in particular of a motor vehicle, in which fuel is injected into a combustion chamber in at least two types of operation and in which an air/fuel mixture flows through a tank ventilating valve and is supplied to the combustion chamber. The invention also relates to a corresponding internal combustion engine and a control unit for such an engine.
A method, an engine, and a control unit of this kind are known, for example, from a so-called gasoline direct injection. In gasoline direct injection, fuel is injected into the combustion chamber of the engine in a homogeneous operation during the intake phase or in a stratified operation during the compression phase. The homogeneous operation is preferably provided for full load operation of the engine, whereas the stratified operation is suitable for idling operation and partial-load operation. The stratified operation is distinguished among other things by means of a motor operation with a surplus of air, i.e. a lean operation. In a direct injection of this kind, the operation is switched between the above-mentioned operation types depending on the operating parameters of the engine.
Operation types of the engine are also understood to mean the homogeneous operation with lambda equals one, a leaner homogeneous operation or homogeneous lean operation, and possibly still other operation types of the engine.
In engines of this kind, it is also known to provide a tank ventilation with which an air/fuel mixture from the fuel tank of the engine can be conveyed through a tank ventilation valve to the combustion chamber of the engine. Tank ventilation can be used to prevent unspent fuel from being emitted into the atmosphere.
The tank ventilation mentioned above must be incorporated into the entire control and/or regulation of the engine. To this end, it is particularly necessary to trigger the tank ventilation valve in such a way that on the one hand, the greatest possible ventilation of the fuel tank is achieved, but that on the other hand, this has no negative influence whatsoever on the pollutant emissions or on the torque desired by the driver of the motor vehicle.
The object of the invention is to produce a method for operating an internal combustion engine with which an optimal tank ventilation can be achieved.
In a method of the type mentioned at the beginning, this object is attained according to the invention by establishing a specific desired fuel rate of the air/fuel mixture flowing through the tank ventilation valve. The stated object is correspondingly attained according to the invention with an internal combustion engine and with a control unit for an engine of this kind.
Using the specific desired fuel rate of the air/fuel mixture flowing through the tank ventilation valve, a variable is produced, with which the respectively current lambda of the engine can be taken into account in the control and/or regulation of the tank ventilation. The tank ventilation can therefore be used not only with a lambda of 1, but also with any air/fuel ratio of the engine. It is therefore possible to use tank ventilation even in a direct-injecting internal combustion engine in which lambda can also not equal 1. The tank ventilation, in particular the triggering of the tank ventilation valve, is then executed based on this specific desired fuel rate.
To this end, it is particularly advantageous if the specific desired fuel rate is regulated to a desired fuel proportion of the air/fuel mixture flowing through the tank ventilation valve. The above-mentioned desired fuel proportion can be inferred in particular from a characteristic field that depends on operating parameters of the engine. The specific desired fuel rate can be weighted with a factor, which represents the charging of an activated charcoal filter that is contained in the fuel tank of the internal combustion engine.
It is also particularly advantageous if an integrator generates the specific desired fuel rate, if the specific desired fuel rate is compared to the desired fuel proportion, and if the comparison result is conveyed back to the integrator. As a result, in the final analysis, the comparison result is corrected by means of the integrator. The specific desired fuel rate is consequently regulated to the specific fuel proportion. As mentioned above, the specific desired fuel rate and therefore the entire above-described regulation can be used under all air/fuel conditions of the internal combustion engine. The known regulation is therefore not limited to a lambda equal to 1.
In an advantageous modification of the invention, a desired through flow factor of the air/fuel mixture flowing through the tank ventilation valve is generated and damped. The desired through flow factor approximately represents the quotient of the desired through flow and the maximal through flow. This desired through flow factor can in the end be used to trigger the tank ventilation valve. Damping the desired through flow factor assures that this factor cannot change abruptly in the positive direction. This achieves the fact that the tank ventilation valve can only open in a delayed fashion. This assures an altogether precise control and/or regulation of the engine takes place.
It is particularly advantageous if the desired through flow factor is generated by a positively fed-back integrator and if the desired through flow factor is limited by a maximal through flow factor. This maximal through flow factor can in particular be determined from the specific desired fuel rate. This achieves the fact that the desired through flow factor can only be opened in a delayed fashion, but can be shut off abruptly. This prevents an abrupt opening of the tank ventilation valve, but at the same time permits the tank ventilation valve to close abruptly.
In another advantageous modification of the invention, a desired mass flow through the tank ventilation valve is generated and damped. This once again achieves the fact that the desired mass flow cannot abruptly change in the positive direction. Therefore positive jumps are reliably prevented within the scope of the control and/or regulation of the entire internal combustion engine.
It is particularly advantageous if the desired through flow factor is converted into a maximal mass flow through the tank ventilation valve, if a positively fed-back integrator generates the desired mass flow, and if the desired mass flow is limited by the maximal mass flow. On the one hand, this achieves the fact that the desired mass flow can only be opened in a delayed fashion. On the other hand, however, it is possible for the desired mass flow to be reduced abruptly and therefore closed.
Particularly significant is the embodiment of the method according to the invention in the form of a computer program that is provided for the control unit of the internal combustion engine. The computer program can run on a computer of the control unit and is suited for carrying out the method according to the invention. In this instance, the invention is embodied by means of the computer program so that this computer program represents the invention in the same way as the method, which the computer program is suited for carrying out. The computer program can be stored in a flash memory. A microprocessor can be provided as the computer.
Other features, possible applications, and advantages of the invention ensue from the following description of exemplary embodiments of the invention, which are depicted in the drawings. All features, which are described or depicted, whether by themselves or in arbitrary combinations, represent subjects of the invention, independent of their combination in the claims or in their interdependency and independent of their formulation or depiction in the specification or in the drawings.